No experience with that bag type but I'd follow the seller recommendation. I used to fly long haul (S America / Europe, S America / N America) frequently with one or two very small children. The bag looks a bit small. I'd recommend a lightweight backpack (leaves you hands free - I kept my personal stuff there) or a large, open-top heavy duty plastic bag or something similar (I used a large Carrefour bag - one of those they sell for hauling groceries to car). The thing is, you will need toys, clothes, books, stickers, nappies, food, milk/juice and need an easy access to everything, also in the dark. Plus a change of clothes for yourself. Forget style - I've been covered in pee, puke, tomato stains and cookie crumbs.
Inside the big bag, I packed feeding kits in ziplock bags (bib, spoon, food, napkins) and a separate small bag that had a few nappies, wipes and clean clothes - one of those could be quickly pulled out without rummaging when needed.
I also took along a pillow if one of the children had to sleep in my lap (airline pillows too small - they helped me sleep better too).
Most useful things to take along: individually packed small milk or juice cartons with straws, cookies, sticker books, iPad, and clean t shirt for yourself.